The molecular basis of colonization of the large intestine by Escherichia coli strains isolated from human feces will be investigated using a mouse model. Experiments are proposed to examine and identify alterations in the surface properties of human fecal E. coli strains which have demonstrated enhanced or impaired ability to colonize the large intestine as a result of limited genetic manipulation (e.g., due to the gain or loss of E. coli plasmids). The surface properties of these cells will be analyzed immunologically and biochemically. The overall objective of the proposed investigation is to identify surface components of E. coli which influence or play a role in colonization.